1. Field of the Invention
The present invention relates to signal processing, and, in particular, to the pre-distortion of signals for transmission, for example, in a wireless communication network, to reduce spurious emissions.
2. Description of the Related Art
Modern wireless communication networks employ complex modulation schemes that necessitate tight control of spurious emissions (sometimes called xe2x80x9cout-of-band emissionsxe2x80x9d) in order to avoid interfering with neighboring carriers and to comply with the requirements of regulatory bodies (e.g., FCC) and standards bodies (e.g. ITU). One source of spurious emissions is the base station transmitter amplifier that is used to amplify signals prior to transmission as wireless (e.g., RF) signals to wireless (e.g., mobile) units in a wireless communication network, such as a cellular voice and/or data network. Prior art techniques for reducing such spurious emissions were able to satisfy previous requirements. However, recent developments in wireless communication networks (e.g., Universal Mobile Telecommunication Service (UMTS)) place additional burden on the base station transmitter amplifier and make it advantageous to reduce the spurious emissions even further.
In the prior art, quasi-linear amplifiers operating in Class A, A/B, or B modes have been modeled as a memory-less non-linearity. (As is known in the art, different classes of operation relate to different quiescent operating points of the amplifiers. In Class A, the transistor is xe2x80x9conxe2x80x9d for the whole cycle of a sine wave input. In Class B, the transistor is xe2x80x9conxe2x80x9d for half the cycle. And, in Class A/B, the transistor is one for more than 50%, but less than 100% of the cycle.) Under the assumption of being modeled as a memory-less non-linearity, the input-output relationship is given by Equation (1) as follows:                                                         y              =                                                G                  ⁡                                      (                                          a                      x                                        )                                                  ⁢                x                                                                                        =                                                (                                                                                    G                        i                                            ⁡                                              (                                                  a                          x                                                )                                                              +                                                                  jG                        q                                            ⁡                                              (                                                  a                          x                                                )                                                                              )                                ⁢                x                                                                                        =                                                                    G                    m                                    ⁡                                      (                                          a                      x                                        )                                                  ⁢                                  ⅇ                                      j                    ⁢                                          xe2x80x83                                        ⁢                                                                  G                        p                                            ⁡                                              (                                                  a                          x                                                )                                                                                            ⁢                x                                                                        (        1        )            
where:
x=axejxcfx86x is the complex baseband representation of the input signal, where ax is the amplitude and xcfx86x is the phase of the input signal x,
y is the complex baseband representation of the output signal, and
Gi(xc2x7),Gq(xc2x7) are arbitrary (i.e., unspecified but suitable) functions of the input signal envelope.
In the above model, it is assumed that the instantaneous gain of the amplifier is solely a function of the instantaneous input envelope. This model can be used to describe both AM/AM and AM/PM distortions that are observed in most amplifiers operating in Class A, A/B, and B modes, where AM means amplitude modulation and PM means phase modulation. A digital pre-distorter can be constructed to linearize an amplifier that is described by the above model.
FIG. 1 shows a block diagram of a conventional amplifier pre-distortion architecture 100 comprising digital pre-distorter 102, which pre-distorts the input signal prior to amplification by memory-less amplifier 104. Pre-distorter 102 implements.
1. A method for computing the instantaneous input envelope ax, and
2. A method of pre-distorting the input signal by multiplying the input signal by a complex gain that is solely a function of the instantaneous input envelope.
If the pre-distorted signal is given by {tilde over (x)}=Gpd(ax)x where Gpd(xc2x7) is the pre-distorter gain, then the input-output relationship for the cascaded pre-distorter and amplifier system can be written according to Equation (2) as follows:                                                                         y                ~                            =                                                G                  ⁡                                      (                                          a                                              x                        _                                                              )                                                  ⁢                                  x                  ~                                                                                                        =                                                G                  (                  "RightBracketingBar"                                ⁢                                                      G                    pd                                    ⁡                                      (                                          a                      x                                        )                                                  ⁢                                  "LeftBracketingBar"                                      a                    x                                    )                                ⁢                                                      G                    pd                                    ⁡                                      (                                          a                      x                                        )                                                  ⁢                x                                                                        (        2        )            
The pre-distorter gain is computed so that the gain of the pre-distorter cascaded with the amplifier is constant. Therefore, for the ideal pre-distorter G(|Gpd(ax)|ax)Gpd(ax)=Gtgt where Gtgt is the target gain for the amplifier.
FIG. 2 is a block diagram of amplifier pre-distortion architecture 100 of FIG. 1, showing further detail on a conventional implementation of pre-distorter 102 in which pre-distorter 102 pre-distorter the input signal prior to amplification by memory-less amplifier 104. In particular,pre-distorter 102 comprises a model 202 of amplifier 104. The input signal x is applied to the amplifier model 202 to generate a model Ĝ(ax)x of the distorted output signal. Difference node 204 generates an estimate of the input-output error based on the input signal x and the modeled distorted output signal Ĝ(ax)x according to Equation (3) as follows:
e{tilde over (y)}x(x)=Ĝ(ax)xxe2x88x92x,xe2x80x83xe2x80x83(3)
where Ĝ(xc2x7) is an estimate of the amplifier gain. The pre-distorted signal {tilde over (x)} is generated at difference node 206 by subtracting the estimated error e{tilde over (y)}x from the input signal x, where the pre-distorted signal {tilde over (x)} is then applied to amplifier 104.
For the sake of clarity, assume that the input and output signals are normalized such that the target gain is unity. Further, assume that the amplifier behaves as a true memory-less linearity. It can be shown that, if amplifier model 202 is constructed accurately, the output {tilde over (y)}of amplifier 104 with the pre-distorted signal input {tilde over (x)}is given by Equation (4) as follows:                                                                         y                ~                            =                                                G                  ⁡                                      (                                          a                                              x                        ~                                                              )                                                  ⁢                                  x                  ~                                                                                                                        xe2x80x83                            ⁢                              ≈                                                                            G                      ⁡                                              (                                                  a                          x                                                )                                                              ⁢                    x                                    -                                      e                                                                  y                        ~                                            ⁢                      x                                                                                                                                                              xe2x80x83                            ⁢                              =                                  x                  +                                      x                    ⁡                                          (                                                                        G                          ⁡                                                      (                                                          a                              x                                                        )                                                                          -                                                                              G                            ^                                                    ⁡                                                      (                                                          a                              x                                                        )                                                                                              )                                                                                                                                                              xe2x80x83                            ⁢                              ≈                x                                                                        (        4        )            
Memory-less amplifier model 202 is only an approximate model for most amplifier systems. Consequently, the optimal pre-distorter constructed from such a model cannot completely linearize most amplifiers.
The problems in the prior art are addressed in accordance with the principles of the present invention by an amplifier model that is able to model the performance of most Class A, A/B, and B amplifiers more accurately than the simple memory-less models of the prior art, such as model 202 of FIG. 2. In certain embodiments of the present invention, a pre-distorter pre-distorts an input signal prior to being applied to an amplifier in order to reduce spurious emissions in the resulting amplified signal. The pre-distorter implements an inverted version of a model of the amplifier that models both the frequency-independent (FI) characteristics of the amplifier as well as the frequency-dependent (FD) characteristics of the amplifier.
In one embodiment, the present invention is a method and apparatus for pre-distorting a signal for amplification. According to the embodiment, an input signal is received, and pre-distortion is applied to the input signal to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate an amplified signal, the pre-distortion reduces spurious emissions in the amplified signal. The pre-distortion is generated using an inverse of a model of the amplifier, where the model comprises a model of frequency-independent characteristics of the amplifier in combination with a model of frequency-dependent characteristics of the amplifier.
In another embodiment, the present invention is an apparatus for pre-distorting a signal for amplification. Each of one or more high-order transfer function elements is configured to apply a transfer function of a different order greater than one to a corresponding distortion product for an input signal. A summation node is configured to sum the input signal and the output of each high-order transfer function element. An inverted transfer function element is configured to apply an inverted first-order transfer function to the output of the summation node. An inverted FI element is configured to invert the frequency-independent gain of the amplifier to generate a pre-distorted signal, such that, when the pre-distorted signal is applied to an amplifier to generate an amplified signal, spurious emissions in the amplified signal are reduced.
In another embodiment, the present invention is a method for generating a model of an amplifier, wherein the model comprises an FI model of frequency-independent characteristics of the amplifier in combination with an FD model of frequency-dependent characteristics of the amplifier, wherein the model is generated by estimating a plurality of transfer functions for the FD model in a sequence corresponding to increasing order of the transfer function.
In another embodiment, the present invention is a method for adaptively updating a pre-distorter configured to pre-distort an input signal for application to an amplifier configured to generate an amplified signal, the pre-distorter comprising at least one look-up table (LUT), wherein the at least one LUT is adaptively updated by minimizing an error between the input signal and a feedback signal based on the amplified signal.